Optimisation of the hatchery production of ballan wrasse (Labrus bergylta) with an emphasis on nutritional and environmental requirements

Ballan wrasse (Labrus bergylta), successfully used as a biological delouser of Atlantic salmon (Salmo salar), are eco-friendlier alternative to the use of chemotherapeutants. However, the use of wild caught specimens, which account for most of the deployed fish, is not sustainable. Therefore, the industry wishes to transition to farmed ballan wrasse, seen as a more sustainable, reliable and predictable source. However, the hatcheries are facing production challenges common to any new marine fish species including low survival, suboptimal growth, poor feed efficiency and lack of robustness that hampers the up-scaling of the production. To increase hatchery productivity, this doctoral project investigated the nutritional and environmental requirements of the species to develop commercial protocols. The first two experimental chapters focused on live feed (e.g. Artemia) enrichment for ballan wrasse larvae especially for phospholipids and essential fatty acids (chapter II) and selenium (chapter III). Chapter II proposes an effective, simple and easy method for simultaneously boosting the phospholipid and essential fatty acids content of the nauplii. In chapter III, an enrichment protocol was created allowing the hatcheries to target specific selenium contents in their nauplii. Then, research looked at the impact of three temperatures (10, 13 and 16 °C) on juveniles feed intake, growth and digestibility (chapter IV). Results showed that growth performances and feed efficiency were greatly improved at 16 °C compared to the lower temperatures. If implemented commercially, this could shorten the on-growing nursery phase by more than four months. Chapter V then studied the effects of dietary protein content (standard 51% vs. high 59 % crude protein) and protein source (fish meal vs plant-based) on the growth performances, feed efficiency and digestibility of juveniles reared at 15 °C. The study showed the successful substitution of fish meal by soy protein concentrate and pea protein, without compromising growth nor fish condition. In addition, fish fed the standard protein diets saw their feed efficiency improved, while signs of mild enteritis were observed in fish fed the high protein diets. Finally, the last experimental study (chapter VI) in this project consisted in a large-scale screening for deformities in the Scottish production of ballan wrasse. Results clearly showed a high prevalence of jaw, operculum and vertebrae malformations as well as the recurrent presence of nephrocalcinosis, at both post-weaning and pre-deployment stages. While this is not unusual in new emerging marine species in aquaculture, aetiology and mitigation measures will need to be developed to ensure both fish welfare and delousing efficacy. Overall, this PhD contributes to the refinement of the rearing protocols and development of suitable diets for ballan wrasse which will contribute to boost hatchery productivity and cleaner fish welfare.